Applicant claims priority of Japanese patent applications, Ser. Nos. 11-271,932, filed Sep. 27, 1999; 11-298,442, filed Oct. 20, 1999; and 11-309,829, filed Oct. 29, 1999.
1. Field of the Invention
This invention relates to a valve adjustment limiter cap assembly, and more particularly to a carburetor valve adjustment limiter cap assembly for an internal combustion engine.
2. Background of the Invention
Government agencies of an increasing number of countries are applying exhaust emission control regulations to protect the environment. These regulations are being applied to all combustion engines including portable or two cycle engines used in common equipment such as chain saws, lawn mowers and hedge trimmers. One means of limiting excessive exhaust emissions in a small two cycle engine is to restrict the maximum amount of fuel delivered to the combustion chamber. This maximum fuel amount is pre-set on each individual engine by the engine manufacturer with the understanding that the end user requires some adjustment capability to meet changing work conditions and environmental factors such as altitude. The higher the altitude, the lower the air density, and the leaner the fuel to air ratio necessary to operate the engine. The user of the engine must therefore be able to adjust the fuel to air mixture ratios and may do so via low and high speed needle valves protruding from the carburetor.
Not only is it desirable to limit the richness of the fuel to air mixture because of exhaust emission regulatory concerns, but the engine manufacturer of a two cycle engine product also wants to restrict minimum amounts of fuel, or the leanness of the fuel to air mixture. Often a user will desire more power from a two cycle engine and will attempt to operate the engine in an ultra-lean state. This will deprive a two cycle engine of proper cooling and will lead to engine damage and warranty problems. Therefore, the caps are designed not only to restrict the carburetor to a maximum amount of fuel, but also to restrict the carburetor to a minimum amount of fuel.
Limiter caps secured to the projecting ends of the low and high speed needle valves are commonly used to restrict the user from demanding too much fuel from a carburetor which could exceed regulatory emission limits. The user purchases the engine already factory set to a maximum fuel amount, adequate for operation in low lying areas. Should the engine be utilized in a high altitude area, the user can still decrease the amount of fuel supplied to compensate for the lower air density.
In a conventional needle valve fuel limiter cap of the carburetor, the cap has a single tab radially projecting outward to engage a stop or an adjacent cap. The single tab limits rotation of the needle valve in both the fuel rich and fuel lean directions and thereby limits fuel adjustment capability. When both the low and high speed needle valves have limiter caps, the caps typically abut due to physical limitations and the stop for the tab is the adjacent cap. The tabs must therefore be axially offset so as not to obstruct the rotation of the adjacent needle valve. To prevent obstruction, caps are made of different shapes between the low and high speed needle valves.
Because a particular carburetor may be applied to numerous engine applications, setting a specific carburetor to a maximum fuel amount prior to flowing on a specific engine, or within a specific environment such as altitude, is not practical. The limiter cap assembly is therefore supplied in a non-engaged mode and often separate from the carburetor itself. Supplying a carburetor in separate parts contributes to manufacturing or assembly inefficiencies and possible regulatory violations if the caps are never actually engaged to the valves.
A limiter cap assembly is engaged to a low and preferably a high speed valve of a carburetor. The valves are engaged threadably to a carburetor body. Rotation of the valve in one direction increases the fuel to an operating engine and rotation of the valve in the opposite direction decreases the fuel amount. A cylindrical first cap attaches telescopically to a head concentrically formed to a rotating and axial moving shank of the low speed valve. Restricting rotation of the shank and therefore axial movement, are first and second tabs projecting radially outward from a peripheral face of the first cap.
A second cap attaches to the head of the high speed valve and longitudinally abuts and axially aligns to the first cap. The second cap, preferably identical to the first cap, is rotated about 180 degrees relative to the first cap prior to mounting on the valve. The first tab of the first cap is in contact with the peripheral face of the second cap when the low speed valve is in the maximum fuel position. The second tab of the first cap is in contact with the peripheral face of the second cap when the low speed valve is in the minimum fuel position. Preferably, the low speed valve rotates in the same direction as the high speed valve and the first and second tabs of the second cap function the same as the tabs of the first cap.
Preferably, the cap assembly has a lid disposed over the first and second caps. The lid has an inner edge defining an opening whose profile matches the mounting profile of the first and second caps. The caps each have axially aligned trailing projections and axially aligned mid projections. The inner edge of the lid is snap fitted between the trailing and mid projections when the caps are in a provisional mounted position.
Objects, features and advantages of this invention include the use of two identical limiting caps, engaged to respective low and high speed valves. The identical limiting caps also provide a simple and inexpensive means to provisional mount the caps to the lid of a protective housing prior to final flow adjustments of the carburetor.